Here's what might happen if North Korea launched a nuclear weapon

Tensions between North Korea and the US have escalated during Donald Trump's presidency, with threatening tweets and talk about the size of each country's nuclear "button." But what would happen if North Korea actually decided to follow through on its threat and launch a nuke? We spoke with experts on North Korea and missile defense to get a detailed look at the early stages of a nuclear war with the hermit kingdom. Following is a transcript of the video.

"North Korea launched another intercontinental ballistic missile."

"People were told to take cover in basements or in concrete buildings."

"Could this kind of missile now reach cities on the East Coast — possibly New York or DC?"

Chris Snyder: What would happen if North Korea launched a nuclear weapon?

US officials think North Korea has as many as 60 nuclear weapons in its arsenal. And tensions between the two countries have escalated to new levels during the Donald Trump presidency. Remember when Trump compared the size of their nuclear buttons?

So how close are we to an actual nuclear war? It's closer than you might think.

The US is reportedly considering a "bloody nose" attack on North Korea. This could include a manned airstrike or shooting down a missile test launch — basically a warning shot intended to humiliate North Korea.

But this warning could be taken as an act of war.

Jeffrey Lewis is an expert on disarmament, arms control, and nonproliferation. He thinks the bloody-nose tactic could lead to disaster.

Jeffrey Lewis: The problem with the bloody-nose approach is, imagine if you saw a guy holding a gun. Like, if you walk up and punch him in the nose, he might very well shoot you. Because he doesn't know that it's only gonig be one punch in the nose.

Snyder: It's also possible that a Trump tweet could push North Korea over the edge.

Lewis: You could imagine that in the middle of a crisis, where there's a lot of stress and confirmation bias and pressure, a poorly worded tweet might be interpreted as the start of an attack.

Snyder: North Korea may also strike first, but that's less likely. At least, that's what former missile-launch officer Bruce Blair thinks.

Bruce Blair: I would be willing to imagine that he would not launch a nuclear weapon at the United States or our allies unprovoked, because he knows that would be the absolute end of his regime.

Lewis: They are worried that the Kim family will suffer the same fate as Saddam [Hussein] and his family or maybe Muammar Gaddafi.

Snyder: Regardless of the trigger, Lewis says North Korea's attack would be in two phases. In the first, North Korea would look to wipe out nearby US defenses. They would launch nuclear missiles at US forces based in Japan and South Korea.

Lewis: When they talk about nuclear weapons, they say the mission of their nuclear weapons is to deter and repel an invasion. The purpose of those nuclear weapons would be to try to destroy the US's ability to invade them. So destroy the airfields where aircraft may operate out of. To destroy any troops that are massed. To try to damage ports.

Snyder: The US has 30,000 troops based throughout South Korea. In the event of an attack, these troops would likely evacuate their bases and help prevent North Korean troops from crossing the border.

Separating North and South Korea is the DMZ — or the Demilitarized Zone.

Tim Wright: This is the Demilitarized Zone between North and South Korea. You can see in the distance over there North Korea, just beyond the rock that juts out.

Snyder: Tilman Ruff was at the DMZ with ICAN — the International Campaign to Abolish Nuclear Weapons. They were there to send a message of peace ahead of the Winter Olympics.

He describes what the mood was like on the border.

Tilman Ruff: Stuff happens quite regularly. They're shooting across the border. North Korean soldiers tried to defect and crossed the border. One of them was successful; one of them was severely injured by being shot repeatedly from the North Korean side.

As soon as war erupts, you know, there are going be millions if not tens of millions of dead in South Korea. Seoul would be ablaze.

Snyder: Over 25 million people live in the Seoul metro area. The city itself is just 35 miles from the DMZ, and there are over 3,200 bomb shelters that can supposedly withstand an artillery attack ... but not necessarily a nuclear or chemical attack.

A single 250-kiloton nuclear bomb over Seoul would cause an estimated 783,000 deaths, and one over Tokyo would cause about 697,000, according to a study by a group called 38 North.

The people of Seoul, however, seem unfazed by the constant threat. Fifty-eight percent of South Koreans don't think North Korea would actually start a war, according to a 2017 survey by Gallup Korea.

"Ironically, living here makes me feel less threatened against [threats]."

"[Nuclear weapons] may be used as a means of threat, but I don't think they would actually use it to start a war."

Snyder: Other potential targets in South Korea could include the port of Busan, the South Korean president's home, the military base Yongsan Garrison, and Camp Humphreys.

Once the nearby US defense is taken out, phase two would be to aim the nukes directly at the United States.

This propaganda photo from 2013 shows a map of potential targets. These include Pacific Command in Hawaii; PACOM's naval ports in San Diego; Barksdale Air Force Base in Louisiana, which is home to the US Air Force's Global Strike Command; and Washington, DC.

It would take about 30 minutes for a missile to reach the US and 10 to 15 to hit Japan or South Korea.

Blair: We wouldn't know until it actually exploded whether it was a conventional warhead, a nuclear, or chemical, or biological warhead.

Snyder: So how would the US respond?

Blair was a missile launch control officer with the US Air Force.

Blair: If Kim Jong Un launched a ballistic missile or more at the United States, we would detect the launch within a minute from satellites in space that have heat sensors.

Snyder: Early-warning teams in Colorado and Omaha, Nebraska, would receive this information as well as reports from ground radar in Alaska, which could identify the path of a missile within two minutes of launch.

Blair: And those crews in the United States would have about three minutes from the time they got the first sensor information until they are supposed to assess whether we are under attack.

Snyder: They alert the Pentagon of the threat, as well as Strategic Command, who would initiate an emergency teleconference with the president and his advisers, including the secretary of state, secretary of defense, and the national security adviser.

A four-star general at Strategic Command would brief the president on the threat and his options.

Blair: He has probably, on average, about six minutes to deliberate and decide what to do. The War Room and the Pentagon would then challenge the president to authenticate using a special code known as the "biscuit."

Snyder: A launch order would be sent to underground and submarine crews that includes the war plan and special authentication codes.

Blair: I was one of them. It would have taken me one minute to carry out that launch order. It's the length of a tweet.

Snyder: All in all, it could be 20 minutes before missiles are launched at North Korea.

If North Korea launches their missile from a submarine, then the entire process needs to be twice as fast.

The US has 44 missile interceptors stationed in California and Alaska, but it's not a guaranteed defense.

Blair: The odds are very low that a single missile could destroy a warhead. So we would have to launch a salvo of our missile interceptors to have any chance of destroying an incoming nuclear warhead.

Snyder: F-35 fighter jets could also be used to respond to and "kill" missiles early in the launch, but we don't yet have this capability in the United States.

North Korea may also fire off decoys to confuse our defenses, while the real nukes hit their targets. Millions of people could die if just one of these nukes hits a major city. While we have an alert system in place, and disaster relief with FEMA, there's very little someone can do to protect themselves in this situation.

Blair: All of the bomb shelters and other protection that we had during the Cold War have all atrophied and basically been shut down. So we're starting all over again almost.

Snyder: North Korea has no missile defense, so the US could very easily wipe out most of the country with its own nukes. But we are more likely to use nonnuclear options to stop them.

Blair: We'd win a conventional war. We don't need nuclear weapons.

Snyder: The US would target its nuclear reactors, missile-production facilities, and launching pads for intercontinental ballistic missiles with stealth aircraft, submarines, and ships.

There are estimates of about 200 launchers spread around North Korea.

The US would also try to take out Kim Jong Un, who would likely be moved to a bunker soon after a US airstrike.

Lewis: With or without nuclear weapons, I don't think Kim gets out of this alive.

Snyder: How would other countries respond?

Japan and South Korea both have ground-based missile-defense systems

Lewis: But those defenses, you know, they don't really have a great operational record.

Snyder: China's involvement would likely depend on who strikes first.

Blair: China would almost certainly come to the defense of North Korea if the United States attacks first. It's also indicated to North Korea, that if North Korea attacks first, it's on its own.

In the case of Russia, I think it would more or less stand pat.

Snyder: While a nuclear war would be devastating, it's a real possibility.

Blair: I think that the odds are very significant that in the next 25 years, nuclear weapons will be used.

Snyder: The Nobel Peace Prize-winning group ICAN is working on a treaty to end nuclear proliferation.

Ruff: I think the more promising approach is really a global one. That doesn't say, "Nuclear weapons are OK for me but not for you. You better get rid of yours." But that says, "Nuclear weapons are not OK for everybody."